What should I do about not being sure if there are pictures of an unveiled girl that I previously published on other websites, after repenting from sin?

It is impermissible to post images of women on the internet. Efforts must be made to remove any such images that have been posted, to the best of one's ability. As for what cannot be found or removed, there is no obligation concerning it, because obligation is tied to capability, as Allah the Almighty says: "So fear Allah as much as you are able."